Regenerable solvent mixtures for acid-gas separation

ABSTRACT

A solvent system for the removal of acid gases from mixed gas streams is provided. Also provided is a process for removing acid gases from mixed gas streams using the disclosed solvent systems. The solvent systems may be utilized within a gas processing system.

FIELD OF THE INVENTION

The present invention relates to solvent systems for the removal ofspecific components of gas streams, as well as devices and methods usingsuch systems. More specifically, the invention can provide for removalof acid gases, such as CO₂, SO₂, COS, CS₂ and NOx. The invention furthercan provide for continuous operation of devices and methods using thesystem. Further, the inventive methods can utilize multipleabsorption/desorption means, including gas absorption/desorption and/orphase-enhanced absorption/desorption.

BACKGROUND OF THE INVENTION

Various strategies are being pursued to minimize the production and/orrelease of undesirable emissions from combustion processes. One suchstrategy is the development of technologies for the specific removal ofacid gases from gas mixtures, such as the exhausts of carbon combustionprocesses. The separation of acid gases, such as CO₂, from gas mixtureshas been carried out industrially for over a hundred years, although noknown process has been used on a large scale such as that required bylarge, industrial power plants. Of the numerous processes used for CO₂separation, current technology mainly focuses on the use of varioussolvents, such as alkali carbonates in the BENFIELD™ Process (UOP, LLC),alcoholamines in the ECONAMINE FG PLUS™ process (Fluor Corporation), andalcohols, diols, and ethers in the RECTISOL® process (Lurgi, GMBH) andthe SELEXOL™ solvent (The Dow Chemical Company). In a typicalsolvent-based process, the gas mixture to be treated is passed through aliquid solvent that interacts with acidic compounds in the gas stream(e.g., CO₂ and SO₂) and separates them from non-acidic components. Theliquid becomes rich in the acid-gas components, which are then removedunder a different set of operating conditions so that the solvent can berecycled for additional acid-gas removal.

Methods for removal of the acid-gas components from rich solventsinvolve pressure and temperature change. Depending on the temperature ofthe gas mixture and the partial pressure of the acid-gas in the mixture,certain solvents are preferred for specific applications. When a solventoperates to interact with an acid-gas by chemical absorption, anexothermic chemical reaction occurs. The reversal of this reactionrequires at least the amount of energy to be added back to the richsolvent that was produced by the forward reaction, not to mention theenergy needed to bring the rich solvent to the temperature wherereversal is appreciable and to maintain conditions to complete thereverse reaction to an appreciable extent. The energy required to obtainpurified acid-gas from the rich solvent contributes to the cost of thepurified product. In particular, the cost of the purified acid-gas hasbecome a significant hurdle for the application of solvent technologiesto fossil-fuel fired power plants for the removal of acid gases fromflue gas.

Non-aqueous solvents have been used to remove CO₂ from natural gasstreams and require less energy for regeneration. Single-componentalcoholic physisorption solvents such as RECTISOL™ and SELEXOL© arecommercially available for CO₂ separation but perform poorly in thehumid, near-ambient pressure conditions associated with flue gas.Alcoholamines and amines have been combined with alcohols, diols, andcyclic carbonates by various researches to form “hybrid solvents” whosereaction mechanisms and kinetics have been studied in the literature.See, Alvarez-Fuster, et al., Chem. Eng. Sci. 1981, 36, 1513; Ali, etal., Separation and Purification Technology 2000, 18, 163; Usubharatana,et al., Energy Procedia 2009, 1, 95; and Park, et al., Sep. Sci.Technol. 2005, 40, 1885. In addition, a process known as the“phase-transitional absorption method” has been disclosed in relation tomethods for deacidizing gaseous mixtures, which generally consists ofthe absorption of acid gases into an “absorbing phase” of less densitythan water consisting of a nitrogenous base and an alcohol, followed bytransfer of the absorbed acid gas into an aqueous “carrier phase”. Theaqueous carrier phase can be regenerated in a regenerator. The processclaims to save energy by absorbing an acid gas at a faster rate than inan absorbing phase alone, and by avoiding the energy required to pump arich absorbing phase to a separate regenerator by utilizing gravity totransfer the acid gas between phases in a single column for absorptionand regeneration.

Another group of non-aqueous liquids which could be developed to addressmany of the problems affecting CO₂ solvents are room temperatureswitchable ionic liquids. These equimolar mixtures of amidine orguanidine nitrogen bases and alcohols are non-ionic room temperatureliquids that react with CO₂ to form room-temperature ionic liquids.Typically, the conductivity of equimolar mixtures increases by one ortwo orders of magnitude when CO₂ is added. Importantly, these solventshave higher CO₂ loadings than some aqueous amines, and are regenerableunder milder conditions. While these solvents are a promisingalternative technology, they are not well-suited for flue gasapplications due to their chemistries with respect to water, whichtypically is a major component of flue gas. CO₂ is captured via theformation of amidinium and guanidinium alkyl carbonate salts derivedfrom the conjugate bases of the deprotonated alcohol components.However, the alkyl carbonate esters are typically hydrolyzed in waterunder basic conditions, resulting in bicarbonate salts.

Accordingly, it would be beneficial to formulate a new solvent systemcapable of effectively removing acid gases from gas streams(particularly water-containing gas streams) and which can be regeneratedat a lower temperature and energy load than the solvents currentlyutilized for such purposes.

SUMMARY OF THE INVENTION

The present disclosure generally provides solvent systems for theremoval of acidic gases, such as CO₂, from a gas stream and methods forremoving acidic gases using such solvent systems. Various solventsystems are described herein that are capable of functioning in thiscapacity.

In one aspect is provided a solvent system comprising a solution formedof an ionic liquid consisting of a nucleophilic amine and a protic,non-aqueous liquid, wherein the ionic liquid reacts with an acidic gasso as to form an ionic solution comprising: 1) a carbamate salt,Zwitterionic sulfamic acid, sulfate salt, or a combination thereof; and2) a protonated weak acid. In certain such solvent systems, thenucleophilic amine is selected from the group consisting of: a primaryamine, a secondary amine, a diamine, a triamine, a tetraamine, apentamine, a cyclic amine, a cyclic diamine, an amine oligomer, apolyamine, an alcoholamine, and mixtures thereof. In certain suchsolvent systems, the protic non-aqueous liquid is a liquid having a pKaof about 8 to about 15. The protic non-aqueous liquid can be, forexample, selected from the group consisting of: a fluorinated alcohol,an optionally substituted phenol; a nitrogen heterocycle, and mixturesthereof. Exemplary protic non-aqueous liquids include, but are notlimited to, 2,2,3,3,4,4,5,5-octafluoropentanol;2,2,3,3-tetrafluoropropanol; 2,2,3,3,3-pentafluoropropanol;2,2,3,3,4,4-hexafluorobutanol; 2,2,2-trifluoroethanol;nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol;1,1,3,3-hexafluoro-2-phenyl-2-propanol; 4-methoxyphenol; 4-ethoxyphenol;2-ethoxyphenol; 4-propoxyphenol; imidazole; benzimidazole; N-methylimidazole; 1-trifluoroacetylimidazole; 1,2,3-triazole; 1,2,4-triazole;2-trifluoromethylpyrazole; 3,5-bistrifluoromethylpyrazole;3-trifluoromethylpyrazole, 2-fluorophenol, 3-fluorophenol,4-fluorophenol, 2-trifluoromethylphenol, 3-trifluoromethylphenol,4-trifluoromethylphenol, and mixtures thereof.

In another aspect is provided a solvent system comprising a solutionformed of:

a mixture of two or more nucleophilic amines and two or more non-aqueousliquids, wherein one or more of the nucleophilic amines have structuressuch that they react with an acidic gas so as to form one or more of acarbamate salt, a mixed carbamate, salt, a Zwitterionic sulfamic acid,and a sulfate salt. In certain embodiments of such solvent systems, thetwo or more nucleophilic amines can be alkyl fluoroaromatic amines. Forexample, the alkyl fluoroaromatic amines can be, for example, selectedfrom the group consisting of 3-fluoro-N-methylbenzylamine,4-fluoro-N-methylbenzylamine, 2-fluorophenethylamine,3-fluorophenethylamine, and 4-fluorophenethylamine. The two or morenon-aqueous liquids used according to certain embodiments of thissolvent system can be, in certain embodiments, selected from the groupconsisting of 2,2,3,3,4,4,5,5-octafluoropentanol,3,3,4,4,5,5,6,6-hexafluorobutanol, and4,4,5,5,6,6,7,7,7-nonafluoroheptanol.

In one aspect is provided a solvent system comprising a solution formedof: a nucleophilic amine; a non-nucleophilic, nitrogenous base; and anon-aqueous liquid, wherein the nucleophilic amine has a structure suchthat it reacts with an acidic gas so as to form a carbamate salt, amixed carbamate salt, a sulfamic acid, a sulfamate, or a sulfate salt,and wherein the non-nucleophilic, nitrogenous base and non-aqueousliquid react to form a mixed carbamate salt, a carbonate ester or aheteroatom analogue of a carbonate ester. In certain such solventsystems, the nucleophilic amine is selected from the group consisting of3-fluoro-N-methylbenzylamine, 4-fluoro-N-methylbenzylamine,2-fluorophenethylamine, 3-fluorophenethylamine, 4-fluorophenethylamine,and mixtures thereof. In certain such solvent systems, thenon-nucleophilic nitrogenous base can be a guanidine or substitutedguanidine. The non-aqueous liquid in this type of solvent system can be,for example, a fluorinated alcohol with five or more carbons.

In an additional aspect is provided a solvent system consisting of: aneat nucleophilic amine with a structure such that it reacts with anacidic gas so as to form an amine carbamate salt, Zwitterionic sulfamicacid, sulfate salt, or mixture thereof. In one specific embodiment, thenucleophilic amine in such a solvent system can be3-fluoro-N-methylbenzylamine.

In a further aspect is provided a solvent system comprising a solutionformed of a mixture of one or more nucleophilic amines and one or morenon-nucleophilic, nitrogenous bases with structures such that they reactwith an acidic gas so as to form carbamates, mixed carbamates, sulfamicacids, sulfate salts, or a mixture thereof. In certain embodiments, sucha solvent system can be such that the one or more nucleophilic aminescomprise primary or secondary amines and/or the one or morenon-nucleophilic, nitrogenous bases comprise tertiary amines, amidines,and/or guanidines (wherein one or more of the primary amines, secondaryamines, tertiary amines, guanidines, and/or amidines can optionally befluorinated). Exemplary primary and secondary amines include, but arenot limited to, 3-fluoro-N-methylbenzylamine,4-fluoro-N-methylbenzylamine, 2-fluorophenethylamine,3-fluorophenethylamine, and 4-fluorophenethylamine.

The nucleophilic amine component in any of these solvent systems can, insome embodiments, be hydrophobic. Where a non-nucleophilic, nitrogenousbase is present, the non-nucleophilic, nitrogenous base can behydrophobic or substantially immiscible with water. Generally, thesolvent systems described herein may, in certain embodiments, besubstantially immiscible with water. For example, in some embodiments,the solvent systems may have a solubility with water of less than about10 g or less than about 20 g of solvent per 100 mL of water. In someembodiments, one or more (including all) components of the solventsystems described herein can be described as hydrophobic, substantiallyimmiscible with water, and/or immiscible with water. The acidic gasesthat can react with the various solvent systems described herein canvary and may comprise, for example, CO₂, SO₂, COS, CS₂, NOx, or acombination thereof. In certain specific embodiments, the acidic gascomprises CO₂ or S02.

In another aspect of the invention is provided a process for the removalof acid gas from a gas stream, comprising contacting an acidgas-containing gas stream with any of the solvent systems describedherein. The gas-containing stream can, in some embodiments, be a mixedgas stream comprising CO₂, SO₂, COS, CS₂, NOx, or a combination thereof.In certain embodiments, the solvent system can tolerate water up to orequal to about 20% water by volume with no degradation of solventperformance. In some embodiments, the acid gas-containing gas streamcomprises water and the water can collect as a phase separate from thesolvent system.

The process can, in certain embodiments, further comprise withdrawing anacid gas-rich solvent and an acid gas-lean gas stream. In someembodiments, the process can further comprise regenerating the acidgas-rich solvent by applying heat to form a regenerated solventcomprising a lower content of acid gas than present in the acid gas-richsolvent. The heat involved in such a process can, for example, bederived from a source selected from the group consisting of low-pressuresteam, hot flue gas, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scheme showing various embodiments of solvent systems andreaction pathway employed for capturing CO₂;

FIG. 2 is a diagram of a reboiler-based system embodied by the presentinvention for the capture and regeneration of acidic gases from a mixedgas stream;

FIG. 3 is a diagram of a reboiler-free system embodied by the presentinvention for the capture of acidic gases from a mixed gas stream;

FIG. 4 is a diagram of a reboiler-assisted system embodied by thepresent invention for the capture of acidic gases from a mixed gasstream;

FIG. 5 is a diagram of a waste heat reboiler system embodied by thepresent invention for the capture of acidic gases from a mixed gasstream; and

FIG. 6 is a diagram of a waste heat utilization system embodied by thepresent invention for the capture of acidic gases from a mixed gasstream.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements. As used in thisspecification and the claims, the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.

In one aspect of the present invention is provided a liquid solventsystem. The solvent system may be used for the separation of acidicgases from gas mixtures. The term “acid gas” or “acidic gas” is intendedto refer to any gas component that can result in formation of an acidwhen mixed with water. Non-limiting examples of acid gases encompassedby the present invention include CO₂, SO₂, COS, CS₂ and NOx. Forsimplicity, the invention is described below in relation specifically toCO₂ and SO₂. It is understood, however, that the present inventionencompasses methods and systems for removal of any acid gas componentfrom a gas stream. In certain embodiments, the solvent system isregenerable in that the acidic gases can be released from the solvent,and the solvent can be reused to separate additional acidic gases fromfurther gas mixtures. In particular embodiments, the solvent system isregenerable at temperatures lower than those typically required forsolvents used for such purposes.

Generally, the solvent systems described herein comprise somecombination of one or more of the following classes of reagents:nitrogenous bases (including nucleophilic amines and non-nucleophilicnitrogenous bases); non-aqueous liquids; protic, non-aqueous liquids;diluents; and/or ionic liquids. In certain aspects, the solvent systemsof the invention comprise a mixture of components from two or more ofthese classes. In certain aspects, the solvent systems of the inventionconsist of one or more components from a single class of these reagents.These classes of reagents are described generally herein. The varioustypes of solvent systems intended to be encompassed by the presentinvention and particularly preferred reagents for each will beseparately described below.

Generally, nitrogenous base components include nucleophilic amines andnon-nucleophilic nitrogenous bases. A nitrogenous base component (i.e.,a nucleophilic amine and/or non-nucleophilic nitrogenous base), is anitrogenous base that reacts according to one or more of the mechanismsprovided herein. For example, the nitrogenous base may react with CO₂and/or with other components of the solvent system according to one ofthe embodiments provided herein. In some embodiments, the nitrogenousbase component(s) (which may be a nucleophilic amine and/ornon-nucleophilic nitrogenous base) can have a pKa of about 8 to about15, about 8 to about 14, about 8 to about 13, about 8 to about 12, about8 to about 11, or about 8 to about 10. In certain embodiments, thenitrogenous base component has a pKa less than about 11. In otherembodiments, the nitrogenous base can have a pKa of between about 12 andabout 15, about 12 to about 14, or about 13 to about 15, such as about12, about 13, about 14, or about 15.

In the solvent systems described herein, the nitrogenous base component(or components) of the solvent systems, where present, is advantageouslyselected such that it has low miscibility with water. In preferredembodiments, the nitrogenous base has higher miscibility with theoptional one or more other components of the solvent system than withwater. In some embodiments, the nitrogenous base component or componentshave high solubility in the optional one or more other components of thesolvent system.

A nucleophilic amine is an amine having a reactive nitrogen center whichbonds with non-hydrogen nuclei under relevant process time-scales andtypical process conditions relevant to the gas mixture subjected totreatment therewith. Nucleophilic amines include, but are not limitedto, primary amines, secondary amines, diamines, triamines, tetraamines,pentamines, cyclic amines, cyclic diamines, amine oligomers, polyamines,alcoholamines, and the like.

A non-nucleophilic nitrogenous base is a nitrogenous base (including butnot limited to, an amine) that acts as a Bronsted base, forming bondswith one or more hydrogen nuclei (protons) under relevant processtime-scale and typical process conditions relevant to the gas mixturesubjected to treatment therewith to give a positively charged nitrogencenter. Non-nucleophilic nitrogenous bases include tertiary amines,guanidines, and amidines and/or analogues thereof.

In certain specific embodiments, various exemplary nitrogenous basesuseful as solvent system components may be selected from the groupconsisting of 1,4-diazabicyclo-undec-7-ene (“DBU”);1,4-diazabicyclo-2,2,2-octane; piperazine (“PZ”); triethylamine (“TEA”);1,1,3,3-tetramethylguanidine (“TMG”); 1,8-diazabicycloundec-7-ene;monoethanolamine (“MEA”); diethylamine (“DEA”); ethylenediamine (“EDA”);1,3-diamino propane; 1,4-diaminobutane; hexamethylenediamine;1,7-diaminoheptane; diethanolamine; diisopropylamine (“DIPA”);4-aminopyridine; pentylamine; hexylamine; heptylamine; octylamine;nonylamine; decylamine; tert-octylamine; dioctylamine; dihexylamine;2-ethyl-1-hexylamine; 2-fluorophenethylamine; 3-fluorophenethylamine;3,5-difluorobenzylamine; 3-fluoro-N-methylbenzylamine;4-fluoro-N-methylbenzylamine; imidazole; benzimidazole; N-methylimidazole; 1-trifluoroacetylimidazole; 1,2,3-triazole; 1,2,4-triazole;and mixtures thereof. Still other nitrogenous bases that may be usedaccording to the present invention include, for example, those disclosedin U.S. Patent Application Publication No. 2008/0058549 to Jessop etal., the disclosure of which is incorporated herein by reference.

A non-aqueous liquid is understood to be a liquid other than water. Incertain situations, the non-aqueous liquid is a protic non-aqueousliquid, which is a liquid with an ionizable hydrogen which readilydissociates in the presence of a non-nucleophilic amine. As such, insome embodiments, the non-aqueous liquid (e.g., protic non-aqueousliquid) is a “relatively acidic component,” understood to mean amaterial having an acidity that is greater than the acidity of water,preferably substantially greater than the acidity of water. For example,in some embodiments, a non-aqueous liquid (e.g., a protic non-aqueousliquid) that is a relatively acidic component can have a pKa of lessthan about 15, less than about 14, less than about 13, less than about12, less than about 11, or less than about 10. In some embodiments, therelatively acidic component has a pKa of about 8 to about 15, 9 to about15, about 10 to about 15, about 11 to about 15, about 12 to about 15,about 13 to about 15, about 8 to about 14, about 8 to about 13, about 8to about 12, or about 8 to about 11, about 9 to about 14, about 9 toabout 13, about 9 to about 12, about 9 to about 11, about 10 to about12, about 10 to about 13, about 10 to about 14, about 11 to about 13, orabout 11 to about 14. Exemplary classes of relatively acidic componentsthat may be used (as non-aqueous liquids or protic non-aqueous liquids)according to certain embodiments of the invention include, but are notlimited to the following: fluorinated alcohols; optionally substitutedphenols; and nitrogen heterocycles (e.g., pyrazoles and imidazoles).Particularly preferred are relatively acidic components selected fromfluorinated alcohols and optionally substituted phenols.

In some embodiments, the solvent systems comprise one or more diluents.A diluent is understood to be a solvent component that does notparticipate in reaction with the other components in the solvent systemto any significant extent. The types of substances that can serve asdiluents in such embodiments include certain non-aqueous liquids(including protic, non-aqueous liquids), as described above. Whether anon-aqueous liquid (including a protic, non-aqueous liquid) can serve asa diluent depends upon the additional component(s) of the solvent systemwherein it is used. Non-aqueous liquids (including protic, non-aqueousliquids) are considered to be reactive components of the solvent systemsdescribed herein unless otherwise stated. Diluents may, in someembodiments, be relatively acidic components. Exemplary classes ofrelatively acidic components that may be used as diluents according tocertain embodiments of the invention include, but are not limited to thefollowing: fluorinated alcohols; optionally substituted phenols; andnitrogen heterocycles (e.g., pyrazoles and imidazoles).

In some embodiments, a diluent can have a pKa of less than about 15,less than about 14, less than about 13, less than about 12, less thanabout 11, or less than about 10. In some embodiments, the diluent has apKa of the alcohol component is about 6 to about 15, about 7 to about15, about 8 to about 15, about 9 to about 15, about 6 to about 14, about7 to about 14, about 8 to about 13, about 9 to about 13, about 6 toabout 12, about 7 to about 12, about 8 to about 12, about 9 to about 12,about 6 to about 11, about 7 to about 11, about 8 to about 11, about 9to about 11, about 6 to about 10, about 7 to about 10, or about 8 toabout 10. In other embodiments, a non-aqueous liquid acting as a diluentis not a relatively acidic component, and does not have a pKa that fallswithin the ranges noted above. For example, the diluent may, in certainembodiments, have a pKa greater than about 15.

In some embodiments, the diluent is preferably a non-aqueous diluent. Incertain embodiments, the diluent is selected such that it has lowmiscibility with water. For example, in some embodiments, the diluenthas a solubility of less than or equal to about 10 g/100 mL in water at25° C. (i.e., 10 g of solvent per 100 mL of water) or about 20 g/100 mLin water at 25° C. In other embodiments, the diluent has a solubility inwater of less than or equal to about 0.01 g/100 mL, less than or equalto about 0.1 g/100 mL, less than or equal to about 0.5 g/100 mL, lessthan or equal to about 1 g/100 mL, less than or equal to about 1.5 g/100mL, less than or equal to about 2 g/100 mL, less than or equal to about2.5 g/100 mL, less than or equal to about 3 g/100 mL, less than or equalto about 4 g/100 mL, less than or equal to about 5 g/100 mL, less thanor equal to about 6 g/100 mL, less than or equal to about 7 g/100 mL,less than or equal to about 8 g/100 mL, or less than or equal to about 9g/100 mL in water at 25° C. In some embodiments, the diluent iscompletely immiscible with water. Using diluents with low watersolubility may result in solvent systems that display one or more of thefollowing attributes: they may require less energy for regeneration; mayhave high CO₂ loading capacities; may be able to tolerate water in thegas stream; and/or may be able to be separated from water without alarge energy penalty.

Certain specific solvent systems are illustrated in FIG. 1 of thepresent application and are described further below. Additionaldiscussion of solvent components that can be used in certain solventsystems of the present disclosure is provided, for example, inInternational Application No. PCT/US2011/050442 to Lail et al., filedSep. 2, 2011 and PCT/US2011/050452 to Lail et al., filed Sep. 3, 2011,which are incorporated herein by reference. In some embodiments, thesolvent systems described herein are substantially immisible with water,having a solubility at 25° C. of less than or equal to about 10 g ofsolvent per 100 mL of water, less than or equal to about 20 g ofsolvent/100 mL of water, less than or equal to about 9 g of solvent/100mL of water, less than or equal to about 8 g of solvent/100 mL of water,less than or equal to about 7 g of solvent/100 mL of water, less than orequal to about 6 g of solvent/100 mL of water, less than or equal toabout 5 g of solvent/100 mL of water, less than or equal to about 4 g ofsolvent/100 mL of water, less than or equal to about 3 g of solvent/100mL of water, less than or equal to about 2 g of solvent/100 mL of water,less than or equal to about 1 g of solvent/100 mL of water, less than orequal to about 0.5 g of solvent/100 mL of water, less than or equal toabout 0.1 g/100 mL of water, or less than or equal to about 0.01 g/100mL of water. In some embodiments, the solvent system is completelyimmiscible with water. Solvent systems with low water miscibility may,in some embodiments, display one or more of the following attributes:they may require less energy for regeneration; may have high CO₂ loadingcapacities; may be able to tolerate water in the gas stream; and/or maybe able to be separated from water without a large energy penalty. It isnoted that although solvent system components having low miscibilitywith water are preferred, the present invention also encompasses solventsystems wherein one or more of the components of the solvent system areat least partially miscible with water.

The solvent systems described herein may, as noted above, be used forthe removal of one or more acidic gases from a gas stream. In someembodiments, the solvent systems of the present disclosure may beparticularly useful for capturing CO₂ from a gas stream. The gas streammay be a mixed gas stream, having one or more other components inaddition to CO₂. When a solution comprising a solvent system of thepresent invention is purged with a gas mixture containing CO₂, one ormore components of the solvent system undergo a chemical reaction withCO₂, binding the CO₂ in the solution. In some embodiments, the solventsystems of the present invention have high CO₂ loadings. For example,the solvent systems may be useful for capturing or removing greater thanabout 0.05 moles CO₂ per mole of nitrogenous base, greater than about0.1 moles CO₂ per mole of nitrogenous base, greater than about 0.2 molesCO₂ per mole of nitrogenous base, greater than about 0.3 moles CO₂ permole of nitrogenous base, greater than about 0.4 moles CO₂ per mole ofnitrogenous base, greater than about 0.5 moles CO₂ per mole ofnitrogenous base, greater than about 0.6 moles CO₂ per mole ofnitrogenous base, greater than about 0.7 moles CO₂ per mole ofnitrogenous base, greater than about 0.8 moles CO₂ per mole ofnitrogenous base, greater than about 0.9 moles CO₂ per mole ofnitrogenous base, or greater than about 1 mole CO₂ per mole ofnitrogenous base.

In some embodiments, any of the solvent systems described herein istolerant to the presence of water. In certain embodiments, the solventsystem tolerates water up to or equal to about 30% water by volume. Forexample, in some embodiments, the solvent system tolerates up to orequal to about 25% water by volume, up to or equal to about 20%, up toor equal to about 15%, up to or equal to about 10%, up to or equal toabout 5%, up to or equal to about 2%, or up to or equal to about 1%water by volume. In some embodiments, tolerance to the presence of watermeans that there is little to no degradation of the solvent performanceup to the indicated volume of water. In some embodiments, the solventsystem maintains at or near its initial capacity for CO₂ loading up tothe indicated volume of water.

In some embodiments, the solvent system may further comprise one or moreadditional components. The additional components may be added, forexample, to increase the solubility of the captured CO₂ product in thesolvent system, and thus avoid the formation of precipitates. In otherembodiments, however, solids formation may be desirable, and suchformation may be enhanced by altering the concentration of one or moresolvent system components.

In preferred embodiments, the CO₂ captured using the solvent system ofthe present invention may be released to regenerate the solvent systemfor reuse. It is preferred that the solvent system is regenerable (orreaction with the CO₂ is reversible) under mild conditions (e.g., at alow temperature). In some embodiments, the release of CO₂ andcorresponding regeneration of the solvent system is effectuated byheating the solution. When the solution containing bound CO₂ is heated,the chemical reaction is reversed and the CO₂ is released, producing aconcentrated CO₂ stream.

In some embodiments, the present application relates to a solvent systemand process for the removal of CO₂ from a gas stream. The presentinvention applies to any gas stream containing CO₂. For example, inparticular embodiments, the invention relates to a process for theremoval of CO₂ from fossil fuel combustion flue gas, a natural gasmixture, or a mixture of respiration gases from closed environmentscontaining CO₂. The process involves passing the mixed gas streamthrough one of the solvent systems described herein. In someembodiments, the present invention further relates to the regenerationof the solvent system, which releases the CO₂. Several techniques can beemployed to regenerate the solvent. These include, but are not limitedto, thermal swing, partial pressure swing, by flashing, stripping,applying a vacuum, or combinations of, pH swing, or combinations of. Insome embodiments, regeneration of the solvent system involves heatingthe solvent system at a temperature sufficient to release the CO₂. Insome embodiments, the process involves heating the solvent system at atemperature at or below about 200° C., for example, at or below about185° C., at or below about 150° C., or at or below about 125° C. Inpreferred embodiments, the process involves heating the solvent systemat a temperature at or below about 100° C., for example, at atemperature at or below about 95° C., at or below about 90° C., at orbelow about 85° C., at or below about 80° C., at or below about 75° C.,or at or below about 70° C. In some embodiments, the CO₂ may be releasedat ambient temperature. In certain embodiments, the CO₂ is captured in anon-aqueous phase under conditions in which water accumulates as aseparate, lower density phase. This phase can be sent to the regeneratorwith the rich, non-aqueous phase to be regenerated at a lowertemperature than the corresponding rich aqueous phase alone. This can befollowed by phase separation from the lean, regenerated solvent beforebeing sent back to the absorber.

In certain embodiments, at or about 100% of the CO₂ is removed from theCO₂-rich solvent system. However, in other embodiments, less than 100%of the CO₂ is removed from the CO₂-rich solvent system. In preferredembodiments, about 50 to 100% of the captured CO₂ is removed from theCO₂-rich solvent system, preferably about 75% to 100%, about 80% to100%, about 90% to 100%, about 95% to about 100%, or about 98% to 100%.For example, in some embodiments, at least about 98%, 95%, 90%, 85%,80%, 75%, 70%, 60%, or 50% of the captured CO₂ is removed from theCO₂-rich solvent system.

In some embodiments, the removal of CO₂ from gas mixtures containing H₂Oin addition to CO₂ can lead to the accumulation of H₂O in the solventsystem, either as a single phase or biphase solution, depending upon thereaction conditions. As noted above, the presence of H₂O in the solventmixture may be disadvantageous because of an undesirable side reaction,and more energy will be required for solvent regeneration due to thenecessity of removing water from the solvent. Thus, the accumulation ofH₂O in the solvent system may increase the regeneration energy demand,decreasing the efficiency of the regeneration system.

In some embodiments, the process of the present invention provides amethod by which the detrimental effects of H₂O accumulation in thesolvent system may be avoided. For example, the detrimental effect ofH₂O accumulation on the solvent system regeneration energy demand may beminimized, by providing a process by which the CO₂ is captured withinthe solvent system at a temperature greater than the H₂O saturationtemperature of the gas mixture. Additionally, in certain embodiments,the detrimental effect of H₂O accumulation on the solvent systemregeneration energy demand may be minimized by providing a process bywhich the H₂O accumulates as a separate, aqueous phase within thesolvent system. This process involves the use of a solvent system thatexhibits little or no solubility in water. In such a system, water thatcollects is present as a separate phase. The separate, aqueous phase maybe decanted or centrifuged off by mechanical, rather than thermal,processes, minimizing the energy required to maintain an efficient CO₂removal system. For example, as the hydrocarbon chain of aliphaticalcohols is increased in length, the solubility of the alcohol in waterdecreases. This is also true for fluorinated alcohols. For example,2,2,3,3,4,4,5,5-octafluoropentanol (“OFP”) is substantially immisciblewith water. Thus, certain solvent systems described herein comprisingappropriate components may form a biphasic liquid solution when combinedwith water. In such solvent systems, water can be separated from thesolvent system without distillation or the use of a membrane bydecanting or centrifugation of the aqueous layer from the fluorinatedphase. In some embodiments, after removal of the H₂O, the CO₂-richsolvent system can be regenerated at a low temperature with the additionof low boiling diluents to satisfy the partial pressure requirements.The solvent system could thus avoid the added energy penalty associatedwith the distillation of water. By providing a non-aqueous CO₂ absorbingsolvent system with low water solubility, the solvent system has lowerenergy demands and milder regeneration conditions than those of aqueousor high-water affinity CO₂ solvent systems.

In some embodiments, a system for the removal of CO₂ from a gas streamis provided. A schematic of an exemplary system of the present inventionis depicted in FIGS. 2 through 6 . The CO₂ removal system 10 includes anabsorber 12 configured with an inlet to receive a gas stream. The gasstream may come directly from, e.g., a combustion chamber of a boilersystem in a power generation plant. The gas stream may or may not bepassed through other cleaning systems prior to entering the CO₂ removalsystem. The absorber may be any chamber wherein a solvent system for theremoval of CO₂ is contained, having an inlet and outlet for a gasstream, and wherein the gas stream may be brought into contact with thesolvent system. Within the absorber, the CO₂ may be transferred fromgaseous phase to liquid phase according to the principles discussedherein. The absorber may be of any type; for example, the absorber maycomprise a spray-tower absorber, packed-bed absorber (includingcountercurrent-flow tower or cross-flow tower), tray-tower absorber(having various tray types, including bubble-cap trays, sieve trays,impingement trays, and/or float valve trays), venture absorber, orejector absorber. The temperature and pressure within the absorber maybe controlled. For example, in one embodiment, the temperature of theabsorber may be maintained at or near 50-60° C. and the absorber may bemaintained at or near atmospheric pressure. Thus, the absorber may beequipped with a heating/cooling system and/or pressure/vacuum system.

Within the absorber, the gas stream is brought into fluid contact withand passed through a solvent system as described herein. The solventsystem reacts with the CO₂ present in the gas stream, capturing it fromthe remaining components of the gas, and the resulting CO₂-free gasstream is released from the absorber through an outlet. The solventsystem continues to react with entering CO₂ as the mixed gas stream ispassed through, until it becomes “rich” with CO₂. The absorber isoptionally connected to one or more components. For example, theabsorber is preferably configured with a means for routing solvent to aunit wherein water may be decanted, centrifuged, or otherwise removedfrom the system.

At any stage in the process of CO₂ capture, the solvent system may beregenerated. The system therefore includes an optional regenerationsystem 14 to release the captured CO₂ via a separate CO₂ gas stream andthus regenerate the solvent system. The regeneration system isconfigured to receive a feed of “rich” solvent from absorber and toreturn regenerated solvent to the absorber once CO₂ has been separatedfrom the “rich” solvent. The regeneration system may simply comprise achamber with a heating unit to heat the solvent system at a temperaturesufficient to release the gas, along with a release valve to allow theCO₂ to be removed from the regeneration system. It may also be adistillation column and have essentially the same design as describedabove for the absorption column. The regenerator may be optionallyconnected to one or more components. For example, the regenerator ispreferably configured with a means for routing solvent to a unit whereinwater may be decanted, centrifuged, or otherwise removed from thesystem.

The released CO₂ can be separated/withdrawn from the system and outputto storage or for other predetermined uses. The regenerated solventsystem is again ready to absorb CO₂ from a gas stream, and may bedirected back into the absorber.

I. Ionic Liquids Comprising a Nucleophilic Amine and a Protic,Non-Aqueous Liquid

In one aspect of the present disclosure, a solvent system comprising anionic liquid is provided, wherein the ionic liquid is prepared bycombining one or more nucleophilic amines and one or more proticnon-aqueous liquids. An ionic liquid solvent system as described in thissection is a system wherein ions (cations and anions) are present insolution. The components generally have appropriate pKa values so as toform an ionic liquid in which the nucleophilic amine is the cation. Incertain embodiments, a solvent system comprising an ionic liquid atambient temperature (e.g., between about 20° C. and about 25° C.) isprovided. Advantageously, ionic liquid solvent systems as described inthis section can react with an acidic gas so as to form an ionicsolution comprising: 1) a carbamate salt, Zwitterionic sulfamic acid,sulfate salt, or a combination thereof; and 2) a protonated, weak acid.

Nucleophilic amines that can be used to form certain exemplary ionicliquid solvent systems of this type can be primary and/or secondaryamines which have reactive nitrogen centers. A primary amine isunderstood to be a compound of the formula NH₂R, where R can be acarbon-containing group, including but not limited to C₁-C₂₀ alkyl. Asecondary amine is understood to be a compound of the formula NHR₁R₂,wherein R₁ and R₂ are independently carbon-containing groups, includingbut not limited to C₁-C₂₀ alkyl. One or more of the hydrogens on R, R₁,and R₂ may optionally be replaced with one or more substituents. Forexample, one or more of the hydrogens on R, R₁, or R₂ may be replacedwith optionally substituted C₁-C₆ alkyl, optionally substituted C₁-C₆alkoxy, optionally substituted C₂-C₁₀ alkenyl; optionally substitutedC₂-C₁₀ alkynyl; optionally substituted alkylaryl; optionally substitutedarylalkyl; optionally substituted aryloxy; optionally substitutedheteroaryl; optionally substituted heterocycle; halo (e.g., Cl, F, Br,and I); hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F,CH₂CF₃, and CF₂CF₃); halogenated aryl; halogenated alkylaryl;halogenated benzyl; optionally substituted amino; optionally substitutedalkylamino; optionally substituted arylamino; optionally substitutedacyl; CN; NO₂; N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonicacid; sulfonate esters (e.g., methanesulfonyl); phosphonic acid;phosphate; phosphonate; mono-, di-, or triphosphate esters; trityl ormonomethoxytrityl; CF₃S; CF₃SO₂; or silyl (e.g., trimethylsilyl,dimethyl-t-butylsilyl, and diphenylmethylsilyl).

In certain embodiments, primary or secondary amines may be selected fromamines functionalized with fluorine-containing-alkyl-aromatic groups. Inspecific embodiments, the amine may be selected from the groupconsisting of 2-fluorophenethylamine, 3-fluorophenethylamine,4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine,3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine,2-fluorobenzylamine, 3-fluorobenzylamine, 4-fluorobenzylamine,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylamine,2,3-difluorobenzylamine, 2,4-difluorobenzylamine,2,6-difluorobenzylamine, 3,4-difluorobenzylamine3,5-di-fluorobenzylamine, 2-trifluormethylbenzylamine,3-trifluormethylbenzylamine, 4-trifluormethylbenzylamine,D-4-fluoro-alpha-methylbenzylamine, andL-4-fluoro-alpha-methylbenzylamine.

In some embodiments, the nucleophilic amines that can be used in suchsolvent systems can comprise cyclic amines, diamines, primary and/orsecondary alcoholamines. Cyclic amines are amines wherein the nitrogenatom forms part of the ring structure, and may include, but are notlimited to, aziridines, azetidines, pyrrolidines, piperidines,piperazines, pyridines, and pyrimidines. Cyclic amines may comprise oneor more rings and may optionally be substituted with one or moresubstituents as listed above. In some embodiments, the nitrogenous basemay be a diamine. In some embodiments, the nitrogenous base may be aprimary or secondary alcoholamine. Alcoholamines are also known as aminoalcohols and contain both an alcohol and amine group. The amine group ofthe alcoholamine may be any type of amine as disclosed herein. Thenucleophilic amine component is advantageously, in some embodiments,hydrophobic.

Protic non-aqueous liquids that may be utilized to form such ionicliquid solvent systems include, for example, fluorinated alcohols;optionally substituted phenols; and nitrogen heterocycles. Certainprotic non-aqueous liquids are fluorinated alcohols (e.g., a fluorinatedalcohol with five or more carbons, preferably with low water content(e.g., <about 10 wt % water)). Fluorinated alcohols useful according tothe present disclosure may comprise any compound having the formulaR—OH, where R is an alkyl group (e.g., C₁-C₁₀ alkyl, C₁-C₈ alkyl, C₁-C₆alkyl, C₂-C₁₀ alkyl, C₂-C₈ alkyl, C₂-C₆ alkyl, C₃-C₁₀ alkyl, C₃-C₈alkyl, or C₃-C₆ alkyl) and wherein one or more hydrogen atoms of thealkyl group is substituted with fluorine. In some embodiments, thenumber of hydrogen atoms replaced with fluorine can be two, three, four,five, six, seven, eight, nine, or even more as may be deemed useful. Infurther embodiments, one or more of the hydrogen atoms of the alkylgroup may optionally be replaced with one or more other substituents,including, but not limited to, C₁-C₆ alkyl, C₁-C₆ alkoxy, and halosubstituents.

Optionally substituted phenols useful in the invention are understood tomean phenols wherein one or more of the hydrogen atoms on the phenylring may be replaced with a substituent. Non-limiting, exemplaryreplacement groups for one or more of the hydrogen atoms on the phenylring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo. Nitrogen heterocyclesare understood to mean any cyclic compound including at least onenitrogen atom in the ring structure (including but not limited toimidazoles, pyrazoles, and triazoles) and being optionally substitutedsuch that one or more of the hydrogen atoms on the ring structure may bereplaced with a substituent. In certain embodiments, at least onenitrogen atom in the ring structure has an acidic hydrogen atom with apKa lower than about 15 (e.g., between about 8 and about 15).Non-limiting, exemplary replacement groups for one or more of thehydrogen atoms on the ring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo.

In some specific embodiments, the protic non-aqueous liquid can be arelatively acidic component selected from the group consisting of:2,2,3,3,4,4,5,5-octafluoropentanol (“OFP”); 2,2,3,3-tetrafluoropropanol(“TFP”); 2,2,3,3,3-pentafluoropropanol (“PFP”);2,2,3,3,4,4-hexafluorobutanol (“HFB”); 2,2,2-trifluoroethanol (“TFE”);nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol;1,1,3,3-hexafluoro-2-phenyl-2-propanol; 4-methoxyphenol (“4-MeOPh”);4-ethoxyphenol (“4-EtOPh”); 2-ethoxyphenol; 4-propoxyphenol; imidazole;benzimidazole; N-methyl imidazole; 1-trifluoroacetylimidazole;1,2,3-triazole; 1,2,4-triazole; 2-trifluoromethylpyrazole;3,5-bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole,2-fluorophenol, 3-fluorophenol, 4-fluorophenol, 2-trifluoromethylphenol,3-trifluoromethylphenol, 4-trifluoromethylphenol, and mixtures thereof.Advantageously, protic non-aqueous liquids used within the solventsystems described herein can have low water content (e.g., <about 10 wt% water) and/or low water solubility. Typically, the protic, non-aqueousliquids used in this type of solvent system are active components of thesolvent system (i.e., not serving only as diluents).

In ionic liquid solvent systems as described herein, the hydrogen nucleiof the protic non-aqueous liquid is sufficiently ionizable to dissociatefrom the protic non-aqueous liquid and react with the nucleophilic base.Acid gas components, such as CO₂ and SO₂, can be absorbed in such anionic liquid solvent by reversible formation of the protic solvent andformation of a bond between the nucleophilic amine nitrogen andnon-hydrogen, acid-gas nuclei forming for example, amine carbamatesalts, Zwitterions (e.g., Zwitterionic sulfamic acid), sulfamicacids/salts, or a combination thereof. One exemplary solvent system andmechanism of reaction is shown in FIG. 1A).

In this type of solvent system, the absorption of the acid gas componentis advantageously reversible. Upon loss of the acid gas component, theprotic solvent again can donate a proton to the nucleophilic base. Thissolvent system has the advantage of minimizing losses of thenucleophilic amine to vapors due to the low vapor pressure of the ionicliquid salt in an absorption column, for instance, and the low vaporpressure of the carbamate salt in a regenerator section, for instance.

II. Mixtures Containing Two or More Nucleophilic Amines and Two or MoreNon-Aqueous Liquids

In another aspect, a solvent system comprising two or more nucleophilicamines mixed together with two or more non-aqueous liquids over a widerange of component ratios is provided and can be used to separate acidgas components from a gas mixture. The two or more nucleophilic aminesreact with the acid gas components (e.g., CO₂ or SO₂) to form at leastone bond to nitrogen involving a nucleus other than hydrogen. Theproduct formed with CO₂ is a carbamate salt and can consist of a singleamine carbamate structure or of a mixed amine carbamate structure. Incertain embodiments, the solvent system removes CO₂ without anysubstantial formation of a carbonate ester or a heteroatom analogue of acarbonate ester.

Nucleophilic amines that can be used to form certain exemplary solventsystems of this type can be primary and/or secondary amines which havereactive nitrogen centers. A primary amine is understood to be acompound of the formula NH₂R, where R can be a carbon-containing group,including but not limited to C₁-C₂₀ alkyl. A secondary amine isunderstood to be a compound of the formula NHR₁R₂, wherein R₁ and R₂ areindependently carbon-containing groups, including but not limited toC₁-C₂₀ alkyl. One or more of the hydrogens on R, R₁, and R₂ mayoptionally be replaced with one or more substituents. For example, oneor more of the hydrogens on R, R₁, or R₂ may be replaced with optionallysubstituted C₁-C₆ alkyl, optionally substituted C₁-C₆ alkoxy, optionallysubstituted C₂-C₁₀ alkenyl; optionally substituted C₂-C₁₀ alkynyl;optionally substituted alkylaryl; optionally substituted arylalkyl;optionally substituted aryloxy; optionally substituted heteroaryl;optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I);hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, andCF₂CF₃); halogenated aryl; halogenated alkylaryl; halogenated benzyl;optionally substituted amino; optionally substituted alkylamino;optionally substituted arylamino; optionally substituted acyl; CN; NO₂;N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonic acid; sulfonateesters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate;mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF₃S;CF₃SO₂; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, anddiphenylmethylsilyl).

In certain embodiments, primary or secondary amines may be selected fromamines functionalized with fluorine-containing-alkyl-aromatic groups. Inspecific embodiments, the amine may be selected from the groupconsisting of 2-fluorophenethylamine, 3-fluorophenethylamine,4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine,3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine,2-fluorobenzylamine, 3-fluorobenzylamine, 4-fluorobenzylamine,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylamine,2,3-difluorobenzylamine, 2,4-difluorobenzylamine,2,6-difluorobenzylamine, 3,4-difluorobenzylamine3,5-di-fluorobenzylamine, 2-trifluormethylbenzylamine,3-trifluormethylbenzylamine, 4-trifluormethylbenzylamine,D-4-fluoro-alpha-methylbenzylamine, andL-4-fluoro-alpha-methylbenzylamine.

In some embodiments, the nucleophilic amines that can be used in suchsolvent systems can comprise cyclic amines, diamines, primary and/orsecondary alcoholamines. Cyclic amines are amines wherein the nitrogenatom forms part of the ring structure, and may include, but are notlimited to, aziridines, azetidines, pyrrolidines, piperidines,piperazines, pyridines, and pyrimidines. Cyclic amines may comprise oneor more rings and may optionally be substituted with one or moresubstituents as listed above. In some embodiments, the nucleophilicamine may be a diamine. In some embodiments, the nucleophilic amine maybe a primary or secondary alcoholamine. Alcoholamines are also known asamino alcohols and contain both an alcohol and amine group. The aminegroup of the alcoholamine may be any type of amine as disclosed herein.

Preferably, one or both of the nucleophilic amines are non-aqueousand/or are hydrophobic and can advantageously have low water solubility(e.g., <about 10 wt %). Certain exemplary nucleophilic amines useful inthis type of solvent system include, but are not limited to, alkylfluoroaromatic amines such as 3-fluoro-N-methylbenzylamine,4-fluoro-N-methylbenzylamine, 2-fluorophenethylamine,3-fluorophenethylamine, and 4-fluorophenethylamine.

Non-aqueous liquids useful according to this type of solvent system canvary. It is noted that one or more such non-aqueous liquids may, in someembodiments, be a protic, non-aqueous liquid. Preferably, one or both ofthe non-aqueous liquids have low water solubility (e.g., <about 10 wt %)and/or are hydrophobic. In certain embodiments, such non-aqueous liquidscomprise fluorinated alcohols. Fluorinated alcohols useful according tothe invention may comprise any compound having the formula R—OH, where Ris an alkyl group (e.g., C₁-C₁₀ alkyl, C₁-C₈ alkyl, C₁-C₆ alkyl, C₂-C₁₀alkyl, C₂-C₈ alkyl, C₂-C₆ alkyl, C₃-C₁₀ alkyl, C₃-C₈ alkyl, or C₃-C₆alkyl) and wherein one or more hydrogen atoms of the alkyl group issubstituted with fluorine. In some embodiments, the number of hydrogenatoms replaced with fluorine can be two, three, four, five, six, seven,eight, nine, or even more as may be deemed useful. In furtherembodiments, one or more of the hydrogen atoms of the alkyl group mayoptionally be replaced with one or more other substituents, including,but not limited to, C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo substituents.Certain exemplary non-aqueous liquids include, but not limited to,2,2,3,3,4,4,5,5-octafluoropentanol, 3,3,4,4,5,5,6,6-hexafluorobutanol,and 4,4,5,5,6,6,7,7,7-nonafluoroheptanol. In specific embodiments, oneor more of the non-aqueous liquids may be selected from the groupconsisting of toluene, p-xylene, 1-methylnaphthalene,2,4,6-dimethylaminophenol, benzylalcohol, 2,6-dimethylcyclohexanone,3,5-lutidine, cyclohexanone, aniline, pyridine, 2-fluoroacetylphenone,1-fluorodecane, 2,4-difluorobenzophenone,2-fluoro-3-trifluoromethylaniline, 2-fluoroaniline, 4-fluoroaniline,3-trifluoromethylacetophenone, 2-trifluoromethylacetophenone,bis(2,2,2-trifluoroethyl)methylphosphonate,4-fluoro-3-(trifluoromethyl)benzaldehyde and mixtures thereof.

Other exemplary classes of protic non-aqueous liquids that may be usedaccording to this class of solvent systems include, but are not limitedto the following: optionally substituted phenols; and nitrogenheterocycles. Optionally substituted phenols useful in the invention areunderstood to mean phenols wherein one or more of the hydrogen atoms onthe phenyl ring may be replaced with a substituent. Non-limiting,exemplary replacement groups for one or more of the hydrogen atoms onthe phenyl ring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo. Nitrogenheterocycles are understood to mean any cyclic compound including atleast one nitrogen atom in the ring structure (including but not limitedto imidazoles, pyrazoles, and triazoles) and being optionallysubstituted such that one or more of the hydrogen atoms on the ringstructure may be replaced with a substituent. In certain embodiments, atleast one nitrogen atom in the ring structure has an acidic hydrogenatom with a pKa lower than about 15 (e.g., between about 8 and about15). Non-limiting, exemplary replacement groups for one or more of thehydrogen atoms on the ring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo.

In some specific embodiments, the non-aqueous liquid is a relativelyacidic component selected from the group consisting of:2,2,3,3,4,4,5,5-octafluoropentanol (“OFP”); 2,2,3,3-tetrafluoropropanol(“TFP”); 2,2,3,3,3-pentafluoropropanol (“PFP”);2,2,3,3,4,4-hexafluorobutanol (“HFB”); 2,2,2-trifluoroethanol (“TFE”);nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol;1,1,3,3-hexafluoro-2-phenyl-2-propanol; 4-methoxyphenol (“4-MeOPh”);4-ethoxyphenol (“4-EtOPh”); 2-ethoxyphenol; 4-propoxyphenol; imidazole;benzimidazole; N-methyl imidazole; 1-trifluoroacetylimidazole;1,2,3-triazole; 1,2,4-triazole; 2-trifluoromethylpyrazole;3,5-bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole,2-fluorophenol, 3-fluorophenol, 4-fluorophenol, 2-trifluoromethylphenol,3-trifluoromethylphenol, 4-trifluoromethylphenol, and mixtures thereof.

One exemplary combination of two nucleophilic amines and two non-aqueousliquids is shown in FIG. 1B). The combination of mixtures of hydrophobicnucleophilic amines and non-aqueous liquids can, in some embodiments,provide certain advantages as compared with solvents involving only asingle hydrophobic amine and a single non-aqueous liquids (e.g., asdescribed in International Application No. PCT/US2011/050452 to Lail etal., filed Sep. 3, 2011, which is incorporated herein by reference). Insome embodiments, such mixed solvent systems are more desirable becausethey enable control over important solvent properties such as viscosity,heat capacity, reaction heat, water content, and/or may preventformation of precipitates in some non-blended formulations that affectthe performance and cost-effectiveness of an acid gas removal process.

Typically, the non-aqueous liquids used in this type of solvent systemare active components of the solvent system (i.e., not serving only asdiluents). Although the present solvent system is described ascomprising one or more non-aqueous liquids, it is noted that, in arelated embodiment, one or more of the non-aqueous liquids can be adiluent. Thus, the present disclosure also, in certain embodiments,relates to mixtures containing two or more nucleophilic amines and twoor more components selected from the group consisting of non-aqueousliquids and diluents.

III. Mixtures Containing Nucleophilic Amine(s), Non-NucleophilicNitrogenous Base(s), and Non-Aqueous Liquid(s)

In one aspect of the invention, solvent systems can comprise mixtures ofone or more nucleophilic amines, one or more non-nucleophilicnitrogenous bases, and one or more non-aqueous liquids. The propertiesof the solvents are altered in such formulations as compared tonon-blended formulations and can advantageously be used to meet specificprocess requirements for gas treatment. In such an embodiment, thesolvent system may react reversibly with carbon dioxide and other acidgases.

In certain embodiments, nucleophilic amines that can be used in thistype of solvent formulation can be primary and/or secondary amines whichhave reactive nitrogen centers. A primary amine is understood to be acompound of the formula NH₂R, where R can be a carbon-containing group,including but not limited to C₁-C₂₀ alkyl. A secondary amine isunderstood to be a compound of the formula NHR₁R₂, wherein R₁ and R₂ areindependently carbon-containing groups, including but not limited toC₁-C₂₀ alkyl. One or more of the hydrogens on R, R₁, and R₂ mayoptionally be replaced with one or more substituents. For example, oneor more of the hydrogens on R, R₁, or R₂ may be replaced with optionallysubstituted C₁-C₆ alkyl, optionally substituted C₁-C₆ alkoxy, optionallysubstituted C₂-C₁₀ alkenyl; optionally substituted C₂-C₁₀ alkynyl;optionally substituted alkylaryl; optionally substituted arylalkyl;optionally substituted aryloxy; optionally substituted heteroaryl;optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I);hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, andCF₂CF₃); halogenated aryl; halogenated alkylaryl; halogenated benzyl;optionally substituted amino; optionally substituted alkylamino;optionally substituted arylamino; optionally substituted acyl; CN; NO₂;N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonic acid; sulfonateesters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate;mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF₃S;CF₃SO₂; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, anddiphenylmethylsilyl).

In certain embodiments, primary or secondary amines may be selected fromamines functionalized with fluorine-containing-alkyl-aromatic groups. Inspecific embodiments, the amine may be selected from the groupconsisting of 2-fluorophenethylamine, 3-fluorophenethylamine,4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine,3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine,2-fluorobenzylamine, 3-fluorobenzylamine, 4-fluorobenzylamine,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylamine,2,3-difluorobenzylamine, 2,4-difluorobenzylamine,2,6-difluorobenzylamine, 3,4-difluorobenzylamine3,5-di-fluorobenzylamine, 2-trifluormethylbenzylamine,3-trifluormethylbenzylamine, 4-trifluormethylbenzylamine,D-4-fluoro-alpha-methylbenzylamine, andL-4-fluoro-alpha-methylbenzylamine.

In some embodiments, the nucleophilic amines that can be used in suchsolvent systems can comprise cyclic amines, diamines, primary and/orsecondary alcoholamines. Cyclic amines are amines wherein the nitrogenatom forms part of the ring structure, and may include, but are notlimited to, aziridines, azetidines, pyrrolidines, piperidines,piperazines, pyridines, and pyrimidines. Cyclic amines may comprise oneor more rings and may optionally be substituted with one or moresubstituents as listed above. In some embodiments, the nucleophilicamine may be a diamine. In some embodiments, the nucleophilic amine maybe a primary or secondary alcoholamine. Alcoholamines are also known asamino alcohols and contain both an alcohol and amine group. The aminegroup of the alcoholamine may be any type of amine as disclosed herein.

Generally, such compounds can react to form bonds with non-hydrogenatoms in acid gas components. These reactions may result in theformation of, for instance, carbamate salts, mixed carbamate salts,zwitterions, sulfamates, and/or sulfamic acids. It is advantageous forthe nucleophilic amines to have low water content (e.g., <about 10 wt %water) and readily form a separate liquid phase when saturated withwater. Exemplary nucleophilic amines for use in these types of solventsystems include, but are not limited to, alkyl fluoroaromatic aminessuch as 3-fluoro-N-methylbenzylamine, 4-fluoro-N-methylbenzylamine,2-fluorophenethylamine, 3-fluorophenethylamine, and4-fluorophenethylamine.

The non-nucleophilic, nitrogenous base component(s) in this type ofsolvent system can vary. In certain embodiments, non-nucleophilicnitrogenous bases which have low water content (e.g., <about 20% wateror <about 10 wt % water at 25° C.) are used, which will readily form aseparate liquid-phase when combined with water. Advantagoueslytherefore, certain non-nucleophilic, nitrogenous bases useful in suchsolvent systems can be hydrophobic and/or substantially immiscible withwater, where “substantially immiscible with water” is as describedelsewhere in the present application. One exemplary type ofnon-nucleophilic nitrogenous base useful in this type of solvent systemis a guanidine or substituted guanidine (e.g., a fluorinated guanidine).

Guanidines are understood to be compounds of the structure RNC(NR₁R₂)₂,wherein R, R₁, and R₂ are independently H or carbon-containing groups,including but not limited to C₁-C₂₀ alkyl. One or more of the hydrogenatoms on R, R₁, and/or R₂ may optionally be replaced with one or moresubstituents. For example, one or more of the hydrogens on R, R₁, R₂,and R₃ may be replaced with optionally substituted C₁-C₆ alkyl,optionally substituted C₁-C₆ alkoxy, optionally substituted C₂-C₁₀alkenyl; optionally substituted C₂-C₁₀ alkynyl; optionally substitutedalkylaryl; optionally substituted arylalkyl; optionally substitutedaryloxy; optionally substituted heteroaryl; optionally substitutedheterocycle; halo (e.g., Cl, F, Br, and I); hydroxyl; halogenated alkyl(e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, and CF₂CF₃); halogenated aryl;halogenated alkylaryl; halogenated benzyl; optionally substituted amino;optionally substituted alkylamino; optionally substituted arylamino;optionally substituted acyl; CN; NO₂; N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio;sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl);phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphateesters; trityl or monomethoxytrityl; CF₃S; CF₃SO₂; or silyl (e.g.,trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl).

Another type of non-nucleophilic nitrogenous base that may be used insuch solvent systems is an amidine, including but not limited to acarboxamidine/carboximidamide, which is understood to be a compound ofthe structure RC(═NH)NR₁R₂, wherein R, R₁, and R₂ are independently H orcarbon-containing groups, including but not limited to C₁-C₂₀ alkyl. Oneor more of the hydrogen atoms on R, R₁, and/or R₂ may optionally bereplaced with one or more substituents. For example, one or more of thehydrogens on R, R₁, R₂, and R₃ may be replaced with optionallysubstituted C₁-C₆ alkyl, optionally substituted C₁-C₆ alkoxy, optionallysubstituted C₂-C₁₀ alkenyl; optionally substituted C₂-C₁₀ alkynyl;optionally substituted alkylaryl; optionally substituted arylalkyl;optionally substituted aryloxy; optionally substituted heteroaryl;optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I);hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, andCF₂CF₃); halogenated aryl; halogenated alkylaryl; halogenated benzyl;optionally substituted amino; optionally substituted alkylamino;optionally substituted arylamino; optionally substituted acyl; CN; NO₂;N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonic acid; sulfonateesters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate;mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF₃S;CF₃SO₂; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, anddiphenylmethylsilyl).

Exemplary guanidines and amidines include, but are not limited to,1,1,3,3-tetramethylguanidine (“TMG”);N-tert-butyl-1,1,3,3-tetramethylguanidine, diphenylguanidine,ditolylguanidine, 1,8-diazabicyclo(5.4.0)undec-7-ene,1,1,3-trimethyl-3-(2,2,3,3-tetrafluoropropyl)guanidine;1,1,3-trimethyl-3-(2,2,3,3,3-pentafluoropropyl)guanidine;1,3-dimethyl-1,3-bis(2,2,2-trifluoroethyl)guanidine;1,3-bis(2,2,3,3-tetrafluoropropyl)guanidine;1,3-bis(4-fluorophenyl)guanidine; 1,3-bis(3-fluorophenyl)guanidine;1,3-bis(2-fluorophenyl)guanidine;2-(2,2,2-trifluoroethyl)-1,4,5,6,-tetrahydropyrimidine;2-(2,2,3,3-tetrafluoropropyl)-1,4,5,6,-tetrahydropyrimidine;3,3,4,4-tetrafluoro-N,N-dimethylbutanimidamide;3,3,3-trifluoro-N,N-dimethylpropanimidamide; and mixtures thereof. Othernon-nucleophilic, nitrogenous bases can also be used as thenon-nucleophilic, nitrogenous base component of such solvent systems,e.g., including, but not limited to, tertiary amines (e.g., fluorinatedtertiary amines).

Advantageously, non-aqueous liquids used within the solvent systemsdescribed herein can have low water content (e.g., <about 10 wt % water)and/or low water solubility. Exemplary classes of non-aqueous liquidsthat may be used according to this class of solvent systems include, butare not limited to the following: fluorinated alcohols; optionallysubstituted phenols; and nitrogen heterocycles. Particularly preferredaccording to this particular type of solvent system are fluorinatedalcohols (e.g., a fluorinated alcohol with five or more carbons,preferably with low water content (e.g., <about 10 wt % water)).Fluorinated alcohols useful according to the present disclosure maycomprise any compound having the formula R—OH, where R is an alkyl group(e.g., C₁-C₁₀ alkyl, C₁-C₈ alkyl, C₁-C₆ alkyl, C₂-C₁₀ alkyl, C₂-C₈alkyl, C₂-C₆ alkyl, C₃-C₁₀ alkyl, C₃-C₈ alkyl, or C₃-C₆ alkyl) andwherein one or more hydrogen atoms of the alkyl group is substitutedwith fluorine. In some embodiments, the number of hydrogen atomsreplaced with fluorine can be two, three, four, five, six, seven, eight,nine, or even more as may be deemed useful. In further embodiments, oneor more of the hydrogen atoms of the alkyl group may optionally bereplaced with one or more other substituents, including, but not limitedto, C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo substituents.

Optionally substituted phenols useful in the invention are understood tomean phenols wherein one or more of the hydrogen atoms on the phenylring may be replaced with a substituent. Non-limiting, exemplaryreplacement groups for one or more of the hydrogen atoms on the phenylring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo. Nitrogen heterocyclesare understood to mean any cyclic compound including at least onenitrogen atom in the ring structure (including but not limited toimidazoles, pyrazoles, and triazoles) and being optionally substitutedsuch that one or more of the hydrogen atoms on the ring structure may bereplaced with a substituent. In certain embodiments, at least onenitrogen atom in the ring structure has an acidic hydrogen atom with apKa lower than about 15 (e.g., between about 8 and about 15).Non-limiting, exemplary replacement groups for one or more of thehydrogen atoms on the ring include C₁-C₆ alkyl, C₁-C₆ alkoxy, and halo.

In some specific embodiments, the non-aqueous liquid is a relativelyacidic component selected from the group consisting of:2,2,3,3,4,4,5,5-octafluoropentanol (“OFP”); 2,2,3,3-tetrafluoropropanol(“TFP”); 2,2,3,3,3-pentafluoropropanol (“PFP”);2,2,3,3,4,4-hexafluorobutanol (“HFB”); 2,2,2-trifluoroethanol (“TFE”);nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol;1,1,3,3-hexafluoro-2-phenyl-2-propanol; 4-methoxyphenol (“4-MeOPh”);4-ethoxyphenol (“4-EtOPh”); 2-ethoxyphenol; 4-propoxyphenol; imidazole;benzimidazole; N-methyl imidazole; 1-trifluoroacetylimidazole;1,2,3-triazole; 1,2,4-triazole; 2-trifluoromethylpyrazole;3,5-bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole,2-fluorophenol, 3-fluorophenol, 4-fluorophenol, 2-trifluoromethylphenol,3-trifluoromethylphenol, 4-trifluoromethylphenol, and mixtures thereof.Typically, the non-aqueous liquids used in this type of solvent systemare active components of the solvent system (i.e., not serving only asdiluents).

When reacted with an acid gas, such as carbon dioxide, a solventcomprising one or more nucleophilic amines, one or morenon-nucleophilic, nitrogenous bases, and one or more protic non-aqueousliquids will form two products, as shown in FIG. 1C). The product formedby reaction of the nucleophilic amine with carbon dioxide will be anamine carbamate salt. The reaction of the non-nucleophilic nitrogenousbase and the protic non-aqueous liquid with carbon dioxide results inthe formation of a carbonate ester. The solvent system therefore has ahigher theoretical carbon dioxide loading than a pure nucleophilic aminesolvent. Addition of the amine to the non-nucleophilic nitrogenous basesolution improves the solvent system by significantly lowering theviscosity of the viscous ionic liquid. The addition of the nucleophilicamine to the non-nucleophilic nitrogenous base and protic non-aqueousliquid solvent system will improve the segregation of water from thenon-nucleophilic nitrogenous base. The molar ratio of nucleophilicamine(s) to non-nucleophilic nitrogenous base(s) can cover a broadrange. Similarly, the molar ratio of nucleophilic amine(s) tonon-nucleophilic nitrogenous base(s) to protic non-aqueous liquid canalso cover a broad range.

IV. Neat, Hydrophobic, Nucleophilic Amine

In another aspect of the invention, neat, hydrophobic, non-aqueoussolvents can be provided. Specifically, a neat solvent according to theinvention can consist of a single nucleophilic amine. The term “neat” asused herein can mean that no other cosolvent is present in the solventsystem, may mean that little to no other liquid is present in thesolvent system (e.g., including situations wherein the solvent systemcomprises a small amount of undesired water, e.g., <about 10 wt %), ormay mean that no other reactive component is present in the solventsystem (i.e., which could react with the hydrophobic, nucleophilicamine, the acidic gas, or both). A neat hydrophobic nucleophilic aminecan, in some embodiments, comprise a mixture of hydrophobic nucleophilicamines, but preferably comprises a single hydrophobic nucleophilic aminecomponent. In some embodiments, a “neat” hydrophobic, nucleophilic aminesolvent system consists of a neat hydrophobic nucleophilic amine and anacidic gas. Neat, hydrophobic nucleophilic amines can react with acidgas components such as CO₂ and SO₂ to form amine carbamate salts,Zwitterionic sulfamic acids, and/or sulfate salts, and in certainembodiments, no additional diluent is required to prevent precipitateformation.

One exemplary hydrophobic, nucleophilic amine suitable for this purposeis 3-fluoro-N-methylbenzylamine, as shown in FIG. 1D). However, theinvention is not intended to be limiting, and other hydrophobic,nucleophilic amines capable of reacting in this way are intended to beencompassed by the present disclosure.

For example, hydrophobic, nucleophilic amines that can be used to formcertain exemplary neat solvent systems of this type can, in someembodiments, be primary and/or secondary amines which have reactivenitrogen centers. A primary amine is understood to be a compound of theformula NH₂R, where R can be a carbon-containing group, including butnot limited to C₁-C₂₀ alkyl. A secondary amine is understood to be acompound of the formula NHR₁R₂, wherein R₁ and R₂ are independentlycarbon-containing groups, including but not limited to C₁-C₂₀ alkyl. Oneor more of the hydrogens on R, R₁, and R₂ may optionally be replacedwith one or more substituents.

For example, one or more of the hydrogens on R, R₁, or R₂ may bereplaced with optionally substituted C₁-C₆ alkyl, optionally substitutedC₁-C₆ alkoxy, optionally substituted C₂-C₁₀ alkenyl; optionallysubstituted C₂-C₁₀ alkynyl; optionally substituted alkylaryl; optionallysubstituted arylalkyl; optionally substituted aryloxy; optionallysubstituted heteroaryl; optionally substituted heterocycle; halo (e.g.,Cl, F, Br, and I); hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl,CH₂F, CH₂CF₃, and CF₂CF₃); halogenated aryl; halogenated alkylaryl;halogenated benzyl; optionally substituted amino; optionally substitutedalkylamino; optionally substituted arylamino; optionally substitutedacyl; CN; NO₂; N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonicacid; sulfonate esters (e.g., methanesulfonyl); phosphonic acid;phosphate; phosphonate; mono-, di-, or triphosphate esters; trityl ormonomethoxytrityl; CF₃S; CF₃SO₂; or silyl (e.g., trimethylsilyl,dimethyl-t-butylsilyl, and diphenylmethylsilyl).

In certain embodiments, primary or secondary amines may be selected fromamines functionalized with fluorine-containing-alkyl-aromatic groups. Inspecific embodiments, the amine may be selected from the groupconsisting of 2-fluorophenethylamine, 3-fluorophenethylamine,4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine,3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine,2-fluorobenzylamine, 3-fluorobenzylamine, 4-fluorobenzylamine,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylamine,2,3-difluorobenzylamine, 2,4-difluorobenzylamine,2,6-difluorobenzylamine, 3,4-difluorobenzylamine3,5-di-fluorobenzylamine, 2-trifluormethylbenzylamine,3-trifluormethylbenzylamine, 4-trifluormethylbenzylamine,D-4-fluoro-alpha-methylbenzylamine, andL-4-fluoro-alpha-methylbenzylamine.

In some embodiments, the hydrophobic, nucleophilic amines that can beused in such solvent systems can comprise cyclic amines, diamines,primary and/or secondary alcoholamines. Cyclic amines are amines whereinthe nitrogen atom forms part of the ring structure, and may include, butare not limited to, aziridines, azetidines, pyrrolidines, piperidines,piperazines, pyridines, and pyrimidines. Cyclic amines may comprise oneor more rings and may optionally be substituted with one or moresubstituents as listed above. In some embodiments, the nucleophilicamine may be a diamine. In some embodiments, the nucleophilic amine maybe a primary or secondary alcoholamine. Alcoholamines are also known asamino alcohols and contain both an alcohol and amine group. The aminegroup of the alcoholamine may be any type of amine as disclosed herein.Notably, to function as a neat hydrophobic amine solvent, somenucleophilic amines (e.g., cyclic amines) are preferably functionalizedwith fluorine-containing groups.

The neat hydrophobic nucleophilic amine solvent preferably has a lowwater content (e.g., <about 10 wt %) and forms a separate liquid phasewith water. There are several advantages to a non-aqueous solventprocess for acid-gas removal by utilization of a neat hydrophobic aminesolvent. First, the low heat capacity of the neat amine solventsignificantly reduces the sensible heat requirement of the solvent.Second, treatment of process water that has come in contact with thesolvent will be simplified due to the reduction in the number ofcomponents in the solvent mixture. A relatively small set of secondaryamines do not require diluents to avoid precipitate formation uponreaction with acid gases (neat nucleophilic amines). Of this small set,many are not suitable for treatment of industrial acid-gas containinggas streams due to miscibility with water. Since many of the industrialacid-gas containing gas streams (e.g., combustion flue gases, cementkiln gases, natural gas, synthesis gases, etc.) may contain highconcentrations of water (typically about 2-30 vol %), neat secondaryamines having water miscibility will strip water from the gas stream,thus creating a mixture with the water. To avoid this mixture formation,the neat secondary amine is advantageously selected such that it hasvery low water miscibility. As a result, in such embodiments, thesolvent of the acid gas removal process can be considered to consist ofor consist essentially of a single component.

V. Mixtures Containing Nucleophilic Amine(s) and Non-NucleophilicNitrogenous Base(s)

In another aspect of the invention, acid gas components (e.g., carbondioxide), can be separated from gas mixtures using a combination of oneor more nucleophilic amines and one or more non-nucleophilic nitrogenousbases. In some embodiment, no diluents are contained in such solventsystems (e.g., a “neat” mixture of nucleophilic amine(s) andnon-nucleophilic nitrogenous bases is provided). However, embodimentswith one or more added diluents are also encompassed within this classof solvent systems. Preferably, solvent systems comprising anucleophilic amine and non-nucleophilic nitrogenous base comprise amixture of a hydrophobic nucleophilic amine and a hydrophobic,non-nucleophilic nitrogenous base with a total water content of lessthan 10 wt %.

Nucleophilic amines that can be used to form certain exemplary solventsystems of this type can be primary and/or secondary amines which havereactive nitrogen centers. A primary amine is understood to be acompound of the formula NH₂R, where R can be a carbon-containing group,including but not limited to C₁-C₂₀ alkyl. A secondary amine isunderstood to be a compound of the formula NHR₁R₂, wherein R₁ and R₂ areindependently carbon-containing groups, including but not limited toC₁-C₂₀ alkyl. One or more of the hydrogens on R, R₁, and R₂ mayoptionally be replaced with one or more substituents. For example, oneor more of the hydrogens on R, R₁, or R₂ may be replaced with optionallysubstituted C₁-C₆ alkyl, optionally substituted C₁-C₆ alkoxy, optionallysubstituted C₂-C₁₀ alkenyl; optionally substituted C₂-C₁₀ alkynyl;optionally substituted alkylaryl; optionally substituted arylalkyl;optionally substituted aryloxy; optionally substituted heteroaryl;optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I);hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, andCF₂CF₃); halogenated aryl; halogenated alkylaryl; halogenated benzyl;optionally substituted amino; optionally substituted alkylamino;optionally substituted arylamino; optionally substituted acyl; CN; NO₂;N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonic acid; sulfonateesters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate;mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF₃S;CF₃SO₂; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, anddiphenylmethylsilyl).

In certain embodiments, primary or secondary amines may be selected fromamines functionalized with fluorine-containing-alkyl-aromatic groups. Inspecific embodiments, the amine may be selected from the groupconsisting of 2-fluorophenethylamine, 3-fluorophenethylamine,4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine,3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine,2-fluorobenzylamine, 3-fluorobenzylamine, 4-fluorobenzylamine,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylamine,2,3-difluorobenzylamine, 2,4-difluorobenzylamine,2,6-difluorobenzylamine, 3,4-difluorobenzylamine3,5-di-fluorobenzylamine, 2-trifluormethylbenzylamine,3-trifluormethylbenzylamine, 4-trifluormethylbenzylamine,D-4-fluoro-alpha-methylbenzylamine, andL-4-fluoro-alpha-methylbenzylamine.

In some embodiments, the nucleophilic amines that can be used in suchsolvent systems can comprise cyclic amines, diamines, primary and/orsecondary alcoholamines. Cyclic amines are amines wherein the nitrogenatom forms part of the ring structure, and may include, but are notlimited to, aziridines, azetidines, pyrrolidines, piperidines,piperazines, pyridines, and pyrimidines. Cyclic amines may comprise oneor more rings and may optionally be substituted with one or moresubstituents as listed above. In some embodiments, the nitrogenous basemay be a diamine. In some embodiments, the nitrogenous base may be aprimary or secondary alcoholamine. Alcoholamines are also known as aminoalcohols and contain both an alcohol and amine group. The amine group ofthe alcoholamine may be any type of amine as disclosed herein.

The non-nucleophilic, nitrogenous base component(s) in this type ofsolvent system can vary. In certain embodiments, non-nucleophilicnitrogenous bases which have low water content (e.g., <about 20% wateror <about 10 wt % water at 25° C.) are used, which will readily form aseparate liquid-phase when combined with water. Advantagoueslytherefore, certain non-nucleophilic, nitrogenous bases useful in suchsolvent systems can be hydrophobic and/or substantially immiscible withwater, where “substantially immiscible with water” is as describedelsewhere in the present application. Exemplary types ofnon-nucleophilic nitrogenous base useful in this type of solvent systemare guanidines or substituted guanidines (e.g., fluorinated guanidines),amidines (e.g., fluorinated amidines), or tertiary amines (e.g.,fluorinated tertiary amines).

Guanidines are understood to be compounds of the structure RNC(NR₁R₂)₂,wherein R, R₁, and R₂ are independently H or carbon-containing groups,including but not limited to C₁-C₂₀ alkyl. One or more of the hydrogenatoms on R, R₁, and/or R₂ may optionally be replaced with one or moresubstituents. For example, one or more of the hydrogens on R, R₁, R₂,and R₃ may be replaced with optionally substituted C₁-C₆ alkyl,optionally substituted C₁-C₆ alkoxy, optionally substituted C₂-C₁₀alkenyl; optionally substituted C₂-C₁₀ alkynyl; optionally substitutedalkylaryl; optionally substituted arylalkyl; optionally substitutedaryloxy; optionally substituted heteroaryl; optionally substitutedheterocycle; halo (e.g., Cl, F, Br, and I); hydroxyl; halogenated alkyl(e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, and CF₂CF₃); halogenated aryl;halogenated alkylaryl; halogenated benzyl; optionally substituted amino;optionally substituted alkylamino; optionally substituted arylamino;optionally substituted acyl; CN; NO₂; N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio;sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl);phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphateesters; trityl or monomethoxytrityl; CF₃S; CF₃SO₂; or silyl (e.g.,trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl).

Amidines include, but are not limited to acarboxamidine/carboximidamide, which is understood to be a compound ofthe structure RC(═NH)NR₁R₂, wherein R, R₁, and R₂ are independently H orcarbon-containing groups, including but not limited to C₁-C₂₀ alkyl. Oneor more of the hydrogen atoms on R, R₁, and/or R₂ may optionally bereplaced with one or more substituents. For example, one or more of thehydrogens on R, R₁, R₂, and R₃ may be replaced with optionallysubstituted C₁-C₆ alkyl, optionally substituted C₁-C₆ alkoxy, optionallysubstituted C₂-C₁₀ alkenyl; optionally substituted C₂-C₁₀ alkynyl;optionally substituted alkylaryl; optionally substituted arylalkyl;optionally substituted aryloxy; optionally substituted heteroaryl;optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I);hydroxyl; halogenated alkyl (e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, andCF₂CF₃); halogenated aryl; halogenated alkylaryl; halogenated benzyl;optionally substituted amino; optionally substituted alkylamino;optionally substituted arylamino; optionally substituted acyl; CN; NO₂;N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio; sulfate; sulfonic acid; sulfonateesters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate;mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF₃S;CF₃SO₂; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, anddiphenylmethylsilyl).

Exemplary guanidines and amidines include, but are not limited to,1,1,3,3-tetramethylguanidine (“TMG”);N-tert-butyl-1,1,3,3-tetramethylguanidine, diphenylguanidine,ditolylguanidine, 1,8-diazabicyclo(5.4.0)undec-7-ene,1,1,3-trimethyl-3-(2,2,3,3-tetrafluoropropyl)guanidine;1,1,3-trimethyl-3-(2,2,3,3,3-pentafluoropropyl)guanidine;1,3-dimethyl-1,3-bis(2,2,2-trifluoroethyl)guanidine;1,3-bis(2,2,3,3-tetrafluoropropyl)guanidine;1,3-bis(4-fluorophenyl)guanidine; 1,3-bis(3-fluorophenyl)guanidine;1,3-bis(2-fluorophenyl)guanidine;2-(2,2,2-trifluoroethyl)-1,4,5,6,-tetrahydropyrimidine;2-(2,2,3,3-tetrafluoropropyl)-1,4,5,6,-tetrahydropyrimidine;3,3,4,4-tetrafluoro-N,N-dimethylbutanimidamide;3,3,3-trifluoro-N,N-dimethylpropanimidamide; and mixtures thereof.

A tertiary amine is understood to be a compound of the formula NR₁R₂R₃,wherein R₁, R₂, and R₃ are independently carbon-containing groups,including but not limited to C₁-C₂₀ alkyl. One or more of the hydrogenson R, R₁, R₂, and R₃ may optionally be replaced with one or moresubstituents. For example, one or more of the hydrogens on R, R₁, R₂,and R₃ may be replaced with optionally substituted C₁-C₆ alkyl,optionally substituted C₁-C₆ alkoxy, optionally substituted C₂-C₁₀alkenyl; optionally substituted C₂-C₁₀ alkynyl; optionally substitutedalkylaryl; optionally substituted arylalkyl; optionally substitutedaryloxy; optionally substituted heteroaryl; optionally substitutedheterocycle; halo (e.g., Cl, F, Br, and I); hydroxyl; halogenated alkyl(e.g., CF₃, 2-Br-ethyl, CH₂F, CH₂CF₃, and CF₂CF₃); halogenated aryl;halogenated alkylaryl; halogenated benzyl; optionally substituted amino;optionally substituted alkylamino; optionally substituted arylamino;optionally substituted acyl; CN; NO₂; N₃; CH₂OH; CONH₂; C₁-C₃ alkylthio;sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl);phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphateesters; trityl or monomethoxytrityl; CF₃S; CF₃SO₂; or silyl (e.g.,trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl).

Certain exemplary formulations include, but are not limited to, one ormore primary and/or secondary amines, including alkyl fluoroaromaticamines such as 3-fluoro-N-methylbenzylamine,4-fluoro-N-methylbenzylamine, 2-fluorophenethylamine,3-fluorophenethylamine, and 4-fluorophenethylamine used in combinationwith one or more tertiary amines (e.g., fluorinated tertiary amines),guanidines (e.g., fluorinated guanidines), and/or amidines (e.g.,fluorinated amidines).

In some specific embodiments, this type of solvent system can consist ofa secondary amine and guanidine as shown in FIG. 1E). In certainembodiments, the formulated solvent (i.e., a mixture containingnucleophilic amine(s) and non-nucleophilic nitrogenous base(s)) reactswith carbon dioxide to form a carbamate salt (e.g., a mixed carbamatesalt). In the reaction product, the nucleophilic amine component forms acarbon-nitrogen bond with CO₂ (or another acid gas) and thenon-nucleophilic amine component forms a bond with a hydrogen nucleus(proton). The structure of the product formed is a mixed amine carbamatesalt. The molar ratio of non-nucleophilic amine(s) to nucleophilicamine(s) can cover a broad range. The mixture of nucleophilic withnon-nucleophilic bases may improve the kinetics of carbon dioxideabsorption and increase the carbon dioxide loading at a giventemperature (carbon-dioxide vapor-liquid-equilibrium) due to improvedthermodynamics. Compared to a conventional CO₂ capture from a solutionutilizing a single nucleophilic amine, the mixed solvent in certainembodiments will absorb more CO₂ at slightly higher temperatures, makingthe solvent preferable for separation of CO₂ from gas streams in certaintemperature ranges.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1-31. (canceled)
 32. A solvent system consisting of: a neat nucleophilicamine with a structure such that it reacts with an acidic gas so as toform an amine carbamate salt, Zwitterionic sulfamic acid, sulfate salt,or mixture thereof.
 33. The solvent system of claim 32, wherein thenucleophilic amine is hydrophobic.
 33. The solvent system of claim 32,wherein the nucleophilic amine is 3-fluoro-N-methylbenzylamine.
 34. Thesolvent system of claim 32, wherein the solvent system is substantiallyimmiscible with water.
 35. The solvent system of claim 32, wherein thesolvent system has a solubility with water of less than about 20 g ofsolvent per 100 mL of water.
 36. The solvent system of claim 32, whereinthe acidic gas comprises CO₂, SO₂, COS, CS₂, NOx, or a combinationthereof.
 37. The solvent system of claim 32, wherein the acidic gascomprises CO₂ or SO₂.
 38. A process for the removal of acid gas from agas stream, comprising contacting an acid gas-containing gas stream withthe solvent system of claim
 32. 39. The process of claim 38, furthercomprising withdrawing an acid gas-rich solvent and an acid gas-lean gasstream.
 40. The process of claim 39, further comprising regenerating theacid gas-rich solvent by applying heat to form a regenerated solventcomprising a lower content of acid gas than present in the acid gas-richsolvent.
 41. The process of claim 40, wherein the applied heat isderived from a source selected from the group consisting of low-pressuresteam, hot flue gas, or a combination thereof.
 42. The process of claim38, wherein the gas-containing stream is a mixed gas stream comprisingCO₂, SO₂, COS, CS₂, NOx, or a combination thereof.
 43. The process ofclaim 38, wherein the gas-containing stream comprises an acidic gascomprising CO₂ or SO₂ or a combination thereof.
 44. The process of claim38, wherein the solvent system tolerates water up to or equal to about20% water by volume with no degradation of solvent performance.